A flow-rate control device that is used in a semiconductor manufacturing process is required to quickly track the real flow-rate value to a flow-rate setting value that is set by a user.
In response to this requirement, a flow-rate control device that is shown in Patent Document 1, for example, is provided with a flow-rate sensor, a valve, and a flow-rate control unit that controls the flow rate using the valve, with the flow-rate control unit being configured to control the degree of opening of the valve using two degree-of-freedom control that combines feedback control according to a flow-rate measurement value that is output from the flow-rate sensor and feedforward control according to the flow-rate setting value that is set.
That is, this flow-rate control device is intended to track the real flow-rate value to the flow-rate setting value at high speed using feedforward control, while compensating for overshoot and the like using feedback control.
However, even with the two degree-of-freedom flow-rate control device described in Patent Document 1, it is difficult to ideally shape the response characteristics of the real flow-rate value during the transient response.
The response of the real flow-rate value in the case where a step input is input as the flow-rate setting value will now be specifically considered. Since the flow-rate measurement value that is output from the flow-rate sensor has a time delay relative to the real flow-rate value of the fluid that flows through the flow channel, a large deviation will occur in the early stages of the transient response when the deviation between the flow-rate setting value and the flow-rate measurement value is simply fed back. Accordingly, in early stages of the transient response, the valve will be controlled to be at a larger degree of opening than the fixed degree of opening originally required as specified by feedforward control, and overshoot will occur. Thereafter, the valve will be controlled by feedback control such that the degree of opening is reduced so as to eliminate the overshoot.
Thus, when a time delay exists in the flow-rate measurement value that is measured with the flow-rate sensor, it is difficult to track the real flow-rate value to the flow-rate setting value due to problems such as overshoot occurring or control becoming unstable when gain is increased. Such control-related problems that occur due to time delays become particularly marked in the case where the flow-rate sensor is a thermal flow-rate sensor.
That is, it is not currently possible to limit feedback control to when conditions are suitable in the case of performing two degree-of-freedom flow-rate control.